The present invention relates to a technology for improving image quality by correcting errors in the deposition positions of droplets in a liquid jetting device and a liquid jetting method for depositing the droplets on a recording medium to form dots.
Further, the present invention relates to a technology for enhancing the image quality concerning the resolution of record dots by intentionally shifting the deposition positions of the droplets.
Conventionally, an ink jet printer, which is one of liquid jetting devices, generally includes a head in which nozzles are arranged rectilinearly. Minute ink droplets are jetted out from each of the nozzles of the head toward a recording medium, such as a printing paper, disposed opposite to the nozzle surface, to form roughly circular dots in column direction and row direction, thereby expressing images and characters as dot pictures.
Here, as one of ink jetting systems, there has been known the thermal system for jetting out an ink or inks by use of thermal energy.
The jetting device of the thermal system includes ink chambers for containing an ink as a liquid, heat-generating resistors as energy generation elements provided in the ink chambers, and nozzles for jetting out the ink as droplets. The ink is rapidly heated by the heat generation element, to generate a bubble in the ink on the heat generation element, and an ink droplet is jetted out from the nozzle by the energy at the time of bubble generation.
From the viewpoint of head structure, further, the ink jetting devices are classified into the serial system for printing while moving the head in the width direction of the recording medium, and the line system in which a multiplicity of heads are arrayed in the width direction of the recording medium to constitute a line head corresponding to the width of printing.
In the line system, a technological approach in which the head extending over the entire width of the recording medium is integrally formed by use of a silicon wafer, a glass or the like is impractical, due to the presence of various problems as to manufacturing method, yield, heat generation, cost, etc.
Therefore, as disclosed in Japanese Patent Laid-Open No. 2002-36522 or the like, there has been known a technology in which a plurality of small heads (they also involve various limitations, and a practical limit of the length in the array direction of nozzles is no more than about 1 inch) are aligned so that their ends become continuous with each other, and an appropriate signal processing is applied to the individual heads, thereby recording images continuous with each other over the entire width of the recording medium at the stage of printing on the recording medium.
However, the above-mentioned related art involves the following problems.
First, an ideal condition will be described. FIGS. 8A and 8B schematically illustrate a head array formed by arraying heads in a direction orthogonal to the array direction of nozzles, and the dots formed by droplets jetted out from the nozzles of the heads.
Specifically, FIG. 8A shows that four heads 11A, 11B, 11C and 11D each constructed by arraying nozzles 18 in a row are arrayed in the direction orthogonal to the array direction of the nozzles 18 to constitute a head array including the four heads. Four color inks, i.e., yellow, cyan, magenta, and black inks are individually contained as liquids in ink chambers (not shown), and energy generation elements (not shown) for imparting energy to the liquids in the ink chambers are provided, whereby the four different-colored inks are jetted out as droplets from the nozzles 18 on the basis of each of the heads 11A to 11D.
Here, for convenience, X direction and Y direction are defined as shown in FIG. 8A (hereinafter, this applies also to other figures).
Therefore, in the case of the line system, the heads 11A to 11D are arrayed in respective lengths comparable to the width (in the X direction) of the recording medium, and, without moving the head array composed of the heads 11A to 11D arrayed in the Y direction, the recording medium is fed in the Y direction, whereby printing can be conducted over the whole surface area of the recording medium.
In addition, while the heads arrayed in the Y direction can respectively jet out the four different-colored inks to print color images in this case, more than four colors are used in some other cases.
Incidentally, the nozzles 18 are schematically treated as to number and size, for easier understanding of the illustration.
Besides, in some cases, the inks are jetted from the nozzles not at the same timing but at appropriately staggered timings at the time of printing. In such a case, for preventing errors from being generated in the deposition positions on the recording medium, the positions of the nozzles in the Y direction may be preliminarily corrected according to the jetting timing sequence. For easier understanding of the illustration, here, an example in which the nozzle positions in the Y direction are all on straight lines is illustrated as a representative example.
Incidentally, in the case of the serial system, the head array composed of the heads 11A to 11D performs printing in the width direction (the Y direction) of the recording medium while being moved in the Y direction in FIG. 8A. Then, when the head array has been moved by a one-pass distance, the recording medium is moved by a predetermined amount in the X direction, and the head array repeats the same operation, thereby printing over the whole surface area of the recording medium.
Next, FIG. 8B illustrates the dots D formed by the ideal head array shown in FIG. 8A.
Whether the head array may be of the line system or the serial system, if there is no deflection in the jetting directions at the time of jetting out the ink droplets from the nozzles 18 of the heads, the deposition positions of the droplets correspond to the nozzles 18, so that the dots D aligned in the X direction on the recording medium are also formed in correspondence with the nozzles 18.
Where there is no deflection in the jetting directions, the forming positions of the dots D in the Y direction are determined by the feeding velocity of the recording medium and the jetting timings of the droplets from the nozzles 18 in the line system. In the serial system, the forming positions of the dots D in the Y direction are determined by the moving velocity of the head array and the jetting timings of the droplets from the nozzles 18.
For easier understanding of illustration, here, the dots are schematically treated as to position and size so that they touch each other both in the X direction and in the Y direction.
In response to printing data, the ink droplets are jetted on an on-demand basis independently for each color, and the dots D deposited on the recording medium such as a printing paper are superposed on each other, whereby color printing can be achieved.
In some cases, color printing can be realized by changing not only the color of droplets but also the concentration of droplets, the number of droplets jetted, the amount jetted, the deposition positions of droplets, the deposition area of droplets, or the like. For simplification of description, however, it is assumed here that the dots D formed by the droplets in four colors are equal in size and the number of the droplets jetted from one nozzle in one operation is one.
Then, in the ideal head array shown in FIG. 8A, the dots D formed by the droplets jetted out from the corresponding nozzles 18 (the n-th nozzles as viewed in the X direction) of the heads 11A to 11D are exactly superposed on each other if their positions in the Y direction coincide with each other, as shown in FIG. 8B.
In other words, it is ideal to configure the head array so that the dots D as shown in FIG. 8B will be formed.
In practice, however, it is difficult to constitute the ideal head array as shown in FIG. 8A, due to the problems concerning scattering on a production basis, yield, cost, and the like.
For example, in the head array shown in FIG. 9A, only the head 11C among the heads 11A to 11D has a positional error in the X direction. Due to the positional error, the dots DC formed by the droplets jetted out from the nozzles of the head 11C show positional errors in the X direction as shown in FIG. 9B.
With the head array including such a mispositioned head, intrinsic printing conforming to given printing data cannot be achieved, so that a lowered print quality results.
In addition, as a means for coping with an increase in resolution of record dots desired, it has been practiced to enhance the density of the nozzles arrayed in the heads. However, there is a limitation to the narrowing of the nozzle pitch.
In view of this, in some cases, as shown in FIG. 10A, a plurality of heads (two heads 11A and 11B, in the case of FIG. 10) equal in the pitch of nozzles 18 are aligned, with their pitches staggered from each other (by half pitch, in the case of FIG. 10), i.e., the heads are arranged in a staggered manner, to enhance (in the case of FIG. 10, to double) the resolution.
If the head array is an ideal head array as shown in FIG. 10A, the dots DA and DB formed by the droplets jetted out from the nozzles 18 of the heads 11A and 11B are staggered from each other by half pitch when their positions in the Y direction coincide, as shown in FIG. 10B. In practice, however, it is difficult to realize such an ideal head array, due to the scattering on a production basis and the like.
Accordingly, as represented by the head array shown in FIG. 11A, the nozzle pitches would be staggered halfway and, as a result, the dots DA formed by the droplets jetted out from the nozzles of the head 11A and the dots DB relevant to the head 11B would overlap with each other, as shown in FIG. 11B.
With such a head array, intrinsic printing conforming to given printing data cannot be achieved, so that a lowered print quality results.